G4 MDD noise, but only in OS 9

[torvan]

Okay, I installed OS 9 on my G4MDD using the available CD elsewhere on the site. It works just fine and dandy.

But I have an annoying kind of noise that only happens when booted into OS9. You have the normal background, then this rattling noise comes on for about 5 seconds, then goes away for 30 seconds, then comes back.

Boot into OS X and no rattling noise at all happens. Just the normal roar of the G4 MDD.

Any idea why?

[GaryN]

My guess is that it's your fan. The late software you installed probably included the firmware patch (whose number I don't remember) that keeps the MDD fan from running at full or almost full speed all of the time in OS9. It causes the fan speed to vary cyclically at about the rate you describe. Take a listen (briefly!) with the side door open and see if you can pinpoint it to the fan. If you're lucky, it's just dirty. If it's that dirty, so is the rest of the airflow path and you should clean out the whole insides. OR, it's rattling because it's slightly loose OR when it speeds up it's blowing something around OR it needs replacing.  The problem is that OS9 does not have a fan speed routine suitable for the MDD - the patch is just a band-aid to try to prevent total noise-driven insanity... You don't hear it in OSX because the fan runs more slowly unless / until there's a real overheat potential.